Cartilage regeneration is one of the most widely studied areas in tissue-engineering. Biomaterial-based scaffolds, especially polyethylene glycol (PEG)-based hydrogels, have been established as a conductive environment for chondrogenic differentiation of progenitor cells. However, despite significant progress, one of the major unsolved challenges has been regenerating tissue that mimics the highly organized zonal architecture of articular cartilage, from superficial to middle and deep zones as well as the calcified zone in the osetochondral junction. Few recent studies have attempted to create such spatial organization, but only using zone-specific chondrocytes isolated from native cartilage. Efforts to generate zonally-organized cartilage-like tissue starting from stem or progenitor cell populations are yet to be reported.
Anatomically, articular cartilage consists of four, spatially-distinct zones; the superficial, middle, deep, and calcified layers, characterized by unique extra-cellular matrix (ECM) compositions, mechanical properties and cellular organization. (FIG. 1A) The ECM is primarily composed of type II collagen and proteoglycans whose relative concentrations vary spatially from the superficial to the deep layer leading to varying mechanical properties of each zone. The superficial zone contains high levels of collagen II, with thin fibrils and elongated chondrocytes, both arranged parallel to the surface. The proteoglycan content is lowest in this zone. The transitional zone contains randomly oriented collagen II fibers with larger diameters than the superficial zone. The chondrocytes are more rounded and the proteoglycan concentration increases in this zone. The deep zone contains the highest concentration of proteoglycans and the collagen fibers have the largest diameter. The collagen fibers and spherical chondrocytes are arranged in vertical columns perpendicular to the subchondral plate. Finally, the calcified cartilage zone contains high levels of collagen X and integrates the cartilage to the subchondral bone.